Koen,
In general, you should find the expiration dates on the certificate that you received when you got the certification.
If you lost those, or want to double-check, you can contact Aruba Training via email: arubatraining@hpe.com
The exam pre-requisites are stated on the pages in HPE Certification pages. And yes, for ACCX you will need a valid ACCP certification and for ACMX or ACDX you will need a valid ACMP certification. I found that out myself when I registered for ACCX and got a message that my ACCP was expired, so I had to do the ACCP before the exam. It is strictly enforced.
Also, if you ACMX expires, you should renew with the HPE6-A40 (at moment of writing, described as Prerequisite Path 2 - upgrade from previous versions of Mobility Expert.
For the ACCX or ACDX this renewal exam doesn't exist and officially the full exam has to be re-taken, however, because there is no such an exam, these certifications don't expire till there is such an exam. I think this is not so much clearly stated on these pages, but it's the message I received when I asked on the 3-year anniversary of my own ACDX.
Please check with the training department directly if there is anything unclear.